Tuning the Handedness: Role of Chiral Component in Peptide

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Interface-Rich Materials and Assemblies

Tuning the Handedness: Role of Chiral Component in Peptideappended Bolaamphiphile-based Co-assembled Hydrogels Sagar Biswas, and Apurba K Das Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03651 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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Tuning the Handedness: Role of Chiral Component in Peptide-appended Bolaamphiphile-based Coassembled Hydrogels Sagar Biswas† and Apurba K. Das*† †

Department of Chemistry, Indian Institute of Technology Indore, Indore 453552, India E-mail: [email protected]

Abstract Chirality is the intrinsic property of a molecule which can be tuned by the change in chirality of a molecule or by the addition of a chiral component as an external stimulus. An L-leucine based dipeptide appended succinic acid based bolaamphiphile co-assembled with D-tartaric acid to form supramolecular right handed nanostructured hydrogel whereas L-tartaric acid co-assembled to form supramolecular left handed nanostructured hydrogel. SEM and TEM experiments revealed the right and left handed helical nanofibers, which are responsible for the formation of supramolecular nanostructured hydrogels. The synergistic chiral effect of L-leucine in peptide bolaamphiphile and D/L-tartaric acid plays significant role in bicomponent gelation with helical nanofibers. The first two amino acids attached both the sides of succinic acid moiety act as a tuning button for supramolecular chirality of amino acids/peptides attached with succinic acid based bolaamphiphiles. The second amino acids play role to modulate supramolecular chirality if the first two amino acids act neutrally to the chirality of bolaamphiphiles which were confirmed by CD spectroscopy.

Keywords: Co-assembly, Bicomponent gelation, Chiral stimuli, Supramolecular chirality, Chiral nanostructure

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Introduction Engineering the chiral nanostructures with a desirable handedness is a great deal of attention due to the various applications in materials science,1-5 chemistry6-12 and biology.13-15 Chiral nanostructures exhibit distinguished photophysical properties and adequate biological effects. Peptides self-assemble and possess both molecular as well as supramolecular chirality.16-19 Therefore, the configuration of amino acids in peptides plays a crucial role to achieve selfassembled chiral nanostructures.20-21 A recent study has shown that configuration of terminal hydrophilic amino acid in peptides plays a significant role to control the overall handedness of the peptide amphiphiles.22 The supramolecular chirality could also be induced or tuned by the amino acid based chiral spacer.23 From a detailed survey of the previous reports, a precise observation is noted that right-handed twist is associated with the amphiphiles of amino acids with L-configuration whereas D-amino acids based amphiphiles assemble to produce left-handed chiral nanostructures.24-26 Moreover, left-handed helical twist is also found for L-amino acids based amphiphiles when β-sheet structure is packed with right-handed β-strands along with the hydrogen bonding direction.27-29 Without alteration of building blocks, the tunability of chiral architecture to a desirable handedness (i.e. right-handed: P; left-handed: M) remains a challenge.30 Moreover, the supramolecular chirality is driven by noncovalent interactions and external components as the bicomponent systems work as a remote control for self-assembly and gelation.31-37 Therefore, peptides could rebuild reverse helical nanostructures by interacting with external components (i.e. chiral, achiral or asymmetric molecules) without changing the configuration of amino acids.38 The reverse helical nanostructures were acquired through the coassembly with additives as both shares complementary functionality for hierarchical selfassembly.39


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To date, supramolecular chirality of bolaamphiphiles has attracted the attention of the researchers.40-41 In general, bolaamphiphiles are captivating biomolecules where hydrophobic spacer connects with two polar head groups. Bolaamphiphiles have shown heat sensitive nature to furnish new chiral nanostructures.42 Multiple bolaamphiphiles capped with nucleotides, sugars, amino acids and peptides have been designed to control the helical nanostructures.41,43-45 Bolaamphiphiles self-assembled to form distinct helical nanostructures when it is comprised with an amino acid with different configuration.46 A previous report suggests that aromatic ringfunctionalized external component has a profound impact on helical co-assembly with bolaamphiphiles.29,47-48 As a result, controlled nanostructured hydrogel was achieved.49 The bolaamphiphiles composed of aromatic amino acids form twisted nanoscale materials by the influence of aromatic additives through π-π stacking interaction.29,49 Indeed few aliphatic peptide bolaamphiphiles have been designed to explore their self-assembled helical architectures, however, helix reversal of aliphatic peptide bolaamphiphiles is interesting in hydrogel state as hydrogels exhibit promising properties.50-52 In biological systems, an aliphatic amino acid leucine has a profound impact on folding of proteins.53-55 So, a thorough understanding and controlling the chiral self-assembling behavior of leucine-based peptide bolaamphiphiles are necessary. To tune the chiral nanostructures through hydrogen bonding in gel state, bi-component self-assembled systems were developed with tartaric acid.56 It is also necessary to understand how the configuration of amino acids in peptide bolaamphiphiles influences the bicomponent gelation process as well as the supramolecular chirality of amino acid/peptide functionalized bolaamphiphiles.


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Experimental section Hydrogel preparation: 11.4 mg (20 mM) of 4 was taken in a clean sample vial and 1 mL of sodium phosphate buffer solution (100 mM, pH = 7.4) was added to it. The resultant mixture was sonicated up to 2 minutes to complete the dissolution of the bolaamphiphile and a transparent solution was obtained. 3 mg (20 mM) of solid D/L-tartaric acid was added with the clear solution. Thereafter to obtain a homogeneous mixture, it was again sonicated for 2 minutes and vortexed well. The resultant mixtures were kept for 5 minutes to achieve self-supporting hydrogels 1, 2. Bolaamphiphiles 1, 2, 5-9 were also tried with the same method but gelation was not observed. NaOH / HCl method was also used to prepare the hydrogels of 1-9 at different pHs. However, gelation was not observed with bolaamphiphiles 1-9 at different pHs. 11.4 mg (20 mM) of 4 and 3 mg (20 mM) of solid D/L-tartaric acid were taken in a clean sample vial and 700 μL of water was added to it. The resultant mixture was basified with 0.5 M NaOH to complete the dissolution of the bolaamphiphile and a transparent solution was obtained. Further, the solution was acidified with 0.1 M HCl solution to maintain the pH 7.4. However, gelation was not observed. Similarly, gelation was tried with 1-9 with the same method but gelation was not observed. Materials and methods: All the chemicals and reagents were obtained commercially. All NMR spectra were recorded with 400 MHz Bruker AV 400 NMR. Compounds concentrations were in the range of 1-10 mM in (CD3)2SO and CDCl3. Mass spectra were recorded on Bruker micrOTOF-Q II by positive electrospray ionizations. Rheological study: Rheological study was performed to determine the mechanical properties of the hydrogels. These properties were assessed using an Anton Paar Physica Rheometer (MCR 301, Austria) with parallel plate geometry (25 mm in diameter) and temperature was controlled at 25 °C. The dynamic moduli of the hydrogel were measured as a function of frequency in the range of 0.1100 rad s-1 with a constant strain value 0.1%. Morphological study: Field emission scanning electron microscope (FE-SEM Supra 55 Zeiss) was used for morphological analysis. The solutions of 1-9, hydrogels 1-3 and all other co-assembled systems (5-9) were directly dried on cover slip and coated with gold for SEM analysis with an operating voltage of 5 kV. Transmission electron microscopic images were taken using a PHILIPS electron 4 ACS Paragon Plus Environment

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microscope (model: CM-200), operated at an accelerating voltage of 200 kV. 100 μL of the solution (20 mM) of 4 in phosphate buffer solution (100 mM) was diluted to 200 μL of double distilled water. The hydrogels (100 μL) were also diluted to 200 μL of double distilled water for TEM sample analysis. Dilute solutions of the samples were drop-casted on carbon-coated copper grids (300 mesh) and stained with sodium phosphotungstic acid. Then the grid was dried by slow evaporation in air and then allowed to dry separately in a vacuum at room temperature. FT-IR spectroscopy: All reported FT-IR spectra were recorded with a Bruker (Tensor 27) FT-IR spectrophotometer. The solid-state measurements were performed using the KBr pellet technique with a scan range between 400 and 4000 cm-1 over 64 scans at a resolution of 4 cm-1 and an interval of 1 cm-1. In FT-IR analysis, 4 was directly used and the hydrogels were dried in vacuum to obtain xerogel prior to the experiment. Wide angle X-ray diffraction: The powder X-ray diffraction (PXRD) measurements were carried out using a Bruker D8 Advance X-ray diffractometer. The X-rays were produced using a sealed tube and the wavelength of the X-rays was 0.154 nm (Cu Ka). The X-rays were detected using a fast counting detector based on silicon strip technology (Bruker LynxEye detector). In PXRD analysis, 4 was used directly and the hydrogels (2 mL, 20 mM) were dried under vacuum prior to the PXRD analysis. Circular Dichroism Spectrometer: CD spectra of 1-9 were analyzed with Jasco J-815 Circular Dichroism (CD) spectrometer. The final concentrations of 1-9 were 400 μM for CD experiment. The CD experiments of 1-9 at pHs 6 and 7.4 prepared using NaOH/HCl method were also carried out. In each case, the concentrations were 400 μM. For concentration dependent CD experiment, different bicomponent systems were developed by varying the concentration of D/L-tartaric acid. In each case, the concentration of bolaamphiphile 4 was same (20 mM). The initial concentration of the composite with a proportion of 4 : D/L-tartaric acids were 20 mM : 0 mM; 20 mM : 4 mM; 20 mM : 8 mM; 20 mM : 12 mM; 20 mM : 16 mM; and 20 mM : 20 mM. The composites (4: D/Ltartaric acids) were then diluted in double distilled water to the final concentration of 250 µM : 0 µM; 250 µM : 50 µM; 250 µM : 100 µM; 250 µM : 150 µM; 250 µM : 200 µM and 250 µM : 250 µM for the CD experiments. The other co-assembled systems of 5-9 with D/L-tartaric acid were also carried out at a concentration ratio of 250 µM : 250 µM. The diluted samples were 5 ACS Paragon Plus Environment

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then measured from 250 to 190 nm with 0.1 data pitch, 20 nm min−1 scanning speed, 1 nm bandwidth, and 4 s D.I.T. Results and Discussion In this work, we synthesized a series of dipeptide appended bolaamphiphiles 4-9 which contain D/L-leucine to investigate the effect of peptide sequences with different chirality on bicomponent

hydrogelation (Table 1). L-leucine based dipeptide appended bolaamphiphile 4 (20 mM) does not form hydrogel without the influence of any external component at pH = 7.4.57 Hydrogelation was also tried in water. The bolaamphiphiles are not sufficiently soluble in water. Therefore, NaOH/HCl method was used to enhance the solubility and maintain the pH 7.4. Bolaamphiphiles didn’t form hydrogel at pH 7.4. The hydrogelation was also tried with other bolaamphiphiles 1-9 at a pH of 6. Bolaamphiphiles 1-9 do not produce any gel and exhibit as clear solution.


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Table 1. Molecular structure of bolaamphiphiles 1-9 with absolute configuration of amino acids. CD behavior of 1-9 and physical appearance of 1-9 with addition of D-tartaric acid (DTA) and L-tartaric acid (LTA). CD behaviors of bicomponent systems are also presented.

Compounds

CD spectra

Configuration of amino acids with respect to position 1 2 2¢ 1¢

External component

Physical appearance

CD Spectra of Bicomponent System

DTA Clear Sol (+)Ve LTA Clear Sol (+)Ve DTA Clear Sol (-)Ve 2 * D D * (-)Ve LTA Clear Sol (-)Ve DTA Clear Sol (+)Ve 3 * L D * Silent LTA Clear Sol (-)Ve DTA Hydrogel 1 (+) Ve 4 L L L L (-)Ve LTA Hydrogel 2 (-) Ve DTA Viscous Sol (+) Ve 5 D D D D (+)Ve LTA Viscous Sol (+) Ve DTA Viscous Sol (-) Ve 6 D L L D (-)Ve LTA Viscous Sol (-) Ve DTA Viscous Sol (+) Ve 7 L D D L (+)Ve LTA Viscous Sol (+) Ve DTA Viscous Sol (+) Ve 8 L L D L (+)Ve LTA Viscous Sol (+) Ve DTA Viscous Sol (-) Ve 9 D L D D (-)Ve LTA Viscous Sol (-) Ve *1-3 contains only two amino acids, and there is no second (2,2¢) amino acid. Whereas 4-9 each contains four (4) amino acids. “(-)Ve” signifies negative CD signal whereas “(+)Ve” denoted positive CD signal. For compounds 1-9, the magnitude of CD signals [(-)Ve or (+)Ve] was considered based on the first band from longer wavelength. . 1

*

L

L

*

(+)Ve

Therefore, equimolar (20 mM) of D-tartaric acid, L-tartaric acid and meso-tartaric acid was added separately into the sodium phosphate buffer solution (100 mM, pH = 7.4) of 1-9 to develop the supramolecular chiral nanostructure in hydrogel state. Selectively, 4 forms hydrogel (Table 1) with L/D-tartaric acid (20 mM) and 7 selectively forms hydrogel (Table S1) with meso-tartaric acid (MTA). At higher concentration (30 mM), only 4 forms hydrogel with D/L-tartaric acid. The hydrogelation was not observed for 5-9. However, viscous solution was observed (Table 1, Figure S1). Compounds 1-3 exhibited as transparent solution after addition of L/D-tartaric acid.


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L-Tartaric acid

Growth of fiber

8 nm

Crosslinked P-helical fibers

Hydrogel 1

(b)

(c)

(a) Supramolecular Sphere of compound 4 Growth of fiber

Crosslinked M-helical fibers

Hydrogel 2

Figure 1. (a) Showing the formation of spheres comprised with 4. The spheres ruptured on interaction with Dtartaric acid and L-tartaric acid. (b) Shows the nucleation of the small helical unit with D-tartaric acid and L-tartaric acid. (c) The growth of right-handed helical fiber with D-tartaric acid and left-handed helical fiber with L-tartaric acid. Higher order self-assembly of right-handed and left-handed chiral nanofibers and resulting formation of crosslinked helical nanofibrous hydrogels 1 and 2.

Hydrogelation was confirmed through a test tube inversion method (Figure 1), and the rheological experiments illustrate the mechanical (viscoelastic) properties of the hydrogels.58 A rigid hydrogel is defined in practice of having a storage modulus (G′) higher than the loss modulus (G″) at variable angular frequency regime. The storage moduli of both hydrogels 1 and 2 are higher than the loss moduli throughout the frequency range (Figure S2). The value of the storage modulus (G′) of the hydrogel 1 is greater than the storage modulus (G') of hydrogel 2. The higher storage modulus of hydrogel 1 in comparison to hydrogel 2 signifies that bolaamphiphile 1 performs better co-assembly with D-tartaric acid in comparison to L-tartaric acid. However, both describe the formation of strong and rigid hydrogels. Hydrogel 3 is also considered as a gel from rheological study. However, hydrogel 3 is comparatively weaker as the storage modulus is less in comparison to hydrogels 1, 2. The co-assembled systems of D/Ltartaric acid with bolaamphiphiles 5-9 exhibited as viscous solution or semi gel. Therefore, these 8 ACS Paragon Plus Environment

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co-assembled systems were also studied by rheological experiments. In each case, the storage modulus and loss modulus intersect each other. These data signify that gel was not formed after co-assembly (Figure S2). The co-assembled systems of 8-9 with D/L-tartaric acid were not sufficiently viscous to carry out the frequency sweep experiments. Scanning electron microscopy (SEM) was performed for clear visualization of the formed morphology in hydrogels 1 and 2 (Figure 2) and all other co-assembled systems. Initially, the morphology of the solution of bolaamphiphiles at a pH of 6 was analyzed. Leucine appended bolaamphiphiles 1, 2, 3 do not form any specific morphology whereas dipeptide appended bolaamphiphiles (4-8) form large spherical aggregates (Figure S3). SEM image of hydrogel 1 (4 + DTA) clearly shows multiple small and narrow P-helical fibers with average diameter of 100 nm. Higher order self-assembly forms large right-handed (P-helical) fibers. Hydrogel 2 (4 + LTA) forms left-handed fibers (M-helical), which was visualized from SEM (Figure 2) study. The co-assembled systems are exhibited as viscous solutions but no crosslinked nanostructures are observed from the SEM experiments (Figure S4 a-i). Hydrogel 3 produces nanorods but no chiral twists are developed in the gel state (Figure S4 j). Transmission electron microscopy (TEM) was also performed to know the more insight of the nanostructure of hydrogels. The TEM image of 4 (pH 7.4) shows spherical aggregates with average diameter of 105 nm (Figure S5a). This is attributed to the self-assembly of 4 without D/L-tartaric acid. Hydrogel 1 forms right-handed (P-helical) nanofibers with average diameter of 45 nm (Figure 2, Figure S5). Hydrogel 2 forms helical nanofibers with a left-handed (M-helical) twist having average diameter of 43 nm (Figure 2, Figure S5). From both SEM and TEM images, it was found that the helical fibers were formed only after the co-assembly of 4 with D/L-tartaric acid. To understand the sphere formation, dynamic light scattering (DLS) experiment was performed with a colloidal


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suspension of 4 (20 mM) in phosphate buffer solution (pH 7.4). DLS data gives clear evidence for the formation of spherical aggregates by 4 with an average diameter of 109 nm (Figure S6).

(b)

(a)

P

2 µm

200 nm

(c)

(d) M

200 nm

1 µm

Figure 2. (a), (b) SEM and TEM images of hydrogel 1 showing right-handed (P-helix) nanofiber and (c), (d) are the SEM and TEM images of hydrogel 2 showing left-handed (M-helix) nanofiber.

The mechanistic investigation of the co-assembly in hydrogels was carried out by FT-IR spectroscopy. In native form, 4 shows amide-II and amide-I stretching vibrations at 1550 and 1646 cm-1 (Figure S7).59 The peak at 3312 cm-1 exhibits due to the presence of hydrogen bonded –NH of amide groups. D-tartaric acid shows peak at 1723 cm-1 for C=O stretching frequency and a broad peak from 2100-4000 cm-1 for –OH and –OH of carboxylic acid group. Hydrogels 1 and 2 show peaks at 1619, 1626 and 1533, 1543 cm-1 for amide-I and amide-II respectively (Figure S7a,b). The peaks at ~1619 and 1626 cm-1 correspond to the hydrogen bonded amide stretching frequencies of hydrogels 1 and 2 respectively.60 Small change in stretching frequencies of amideI and amide-II was observed in the co-assembled hydrogels 1 and 2. This signifies better hydrogen bonding between 4 and D-tartaric acid in the hydrogel 1 in comparison to hydrogel 2 (4 with L-tartaric acid) as the peak shifted more in hydrogel 1. The presence of hydrogen bonding 10 ACS Paragon Plus Environment

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was also supported by the appearance of new peaks at 3276 and 3405 cm-1 for hydrogen bonded –NH stretching of amide.61 Similar terminal hydrogen bonding between bolaamphiphile and Dtartaric acid was also reported by Liu et. al.56 The powder X-ray diffraction (PXRD) study was performed to know the detail about the co-assembly occurred between 4 and D/L-tartaric acids in hydrogels (Figure S8). In PXRD, 4 exhibits strong characteristic peaks at 2q of 6.86°, 8.11°, 12.77° and 19.40°. D/L-tartaric acids both show characteristic peaks which are disappeared after formation of hydrogels 1 and 2 (with L-tartaric acid). The peaks at 6.86° and 8.11° in 4 are exhibited for the d spacing of 1.28 and 1.08 nm corresponding to the formation of self-assembled monolayer.62 The hydrogel 1 (with D-tartaric acid) shows peaks at 2q of 5.13°, 12.85°, 21.93° and 33.38° (Figure S8a), which signify the co-assembly in hydrogel state. The hydrogel 2 (with L-tartaric acid) shows peaks at 2q of 5.16°, 22.09° and 33.7° (Figure S8b). The peaks for self-

assembled monolayer are disappeared in hydrogels 1 and 2 and new peaks appear at 5.13°, 5.16° signifying the d spacing of 1.72 nm, 1.71 nm. The d spacing corresponds to the distance between two peptide bolaamphiphiles. In hydrogels 1 and 2, the d spacing value of 0.41 nm signifies the distance between the alkyl groups present in leucine of two bolaamphiphiles.63 Hydrogels 1 and 2 reveal d spacing of 0.26 nm corresponding to the peak at 2q of 33.45° and 33.7° respectively due to the involvement through hydrogen bonding. In hydrogel 1, one additional peak was found at a d spacing of 0.54 nm which is absent in hydrogel 2. This suggests different co-assembly pattern in both the hydrogels. The concentration dependent NMR study shows significant change in the -Hs of D/L-tartaric acid. NH groups of amides of 4 exchanged to ND in presence of D2O. ND peaks remain silent in 1

H NMR experiments. The -Hs of tartaric acid significantly shift to up field in 1H NMR for both

the hydrogels 1, 2. This shift signifies that significant interaction occurs in gel state (Figure S9, S10). 11 ACS Paragon Plus Environment

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(a)

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10 5 0

8 9

(+) CD

-5 -10 -15 -20

200

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Wavelength (nm)

Figure 3. Circular dichroism spectra of 1-9 depending upon the configuration of the amino acids present in bolaamphiphiles. In all the cases, the concentrations of the bolaamphiphiles (1-9) were 400 µM.

To understand the effect of chirality of each amino acid in bolaamphiphile on bicomponent helical nanostructured hydrogel, it is necessary to perform the circular dichroism study. Therefore, the CD experiments, of the synthesized bolaamphiphiles 1-9 were studied thoroughly. 12 ACS Paragon Plus Environment

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Amino acid functionalized bolaamphiphiles 1-3 are the base unit of dipeptide capped bolaamphiphiles. Therefore, at first 1-3 were taken into consideration for CD study. Bolaamphiphile 1 composed with L-leucine (Table 1) exhibits a strong negative peak at 199 nm (Figure 3) due to the transition from π-π* of C=O group of amide and a small positive peak appears at 222 nm corresponding to n-π* transition.64 A CD spectrum of 2 shows the mirror image spectra of 1, as 2 is made of D-leucine (Table 1). When two end of succinic acid possesses L-leucine and D-leucine (3) then it becomes CD silent (Figure 3, Table 1). A strong negative and

positive peak at 197 nm (Figure 3) is observed for 4 and 5 respectively due to the π-π* transition of amide (-CONH) group. Two negative CD bands at 201 nm due to π-π* transition of carbonyl (C=O) group and 222 nm (Figure 3) due to n-π* transition of amide (-CONH) group are observed for 6.61 The similar spectra of 6 with opposite magnitude is observed in case of 7. Bolaamphiphiles 8-9 exhibit L-leucine and D-leucine at first (1,1¢) position respectively (Table 1). A strong negative CD band at 198 nm with a small positive peak appears at 220 nm (Figure 3) for 8 as D-leucine at the second (2,2¢) position. The reverse spectrum of 8 is observed for 9 as the D-leucines are at the second position. The bolaamphiphiles (4,6) with L-leucine at first (1,1¢) position (Table 1) show a predominantly negative cotton effect whatever the configuration of leucine at second (2,2¢) position. Similarly, bolaamphiphiles 5,7 consist of D-leucine at first (1,1') position (Table 1) give positive spectra irrespective of the configuration of second (2,2¢) amino acid. The chirality of leucine at second (2,2') position will be operational for the handedness only if both D-leucine and L-leucine are present at first (1,1') position respectively. Therefore, the overall chirality of 1-9 solely depends on the presence of first amino acids. The CD spectra of 19 were acquired in water medium (Figure S11). However, no significant change is observed. The CD experiments were also carried out to know the self-assembly pattern of 1-9 at lower pH. At pH 6, no significant change is observed from the CD spectra (Figure S12). 13 ACS Paragon Plus Environment

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D/L-tartaric (20 mM) acid integrates into the solution (20 mM) of dipeptide-appended

bolaamphiphiles 4-9. Bolaamphiphile 4 responds to the chiral stimuli D/L-tartaric acid by forming hydrogels and shows complete inversion of CD spectra with D-tartaric acid (Figure 4).

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0 µM LTA 250 µM LTA (-) CD

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0

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LTA 0 µM 50 µM 100 µM 150 µM 200 µM 250 µM

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Figure 4. CD spectra of 4 with variable concentration of D/L-tartaric acid. (a) Shows complete inversion of CD spectra after addition of 250 µM of D-tartaric acid (DTA). (b) Shows small change in CD spectra on addition of Ltartaric acid (LTA). In every case, the concentration of the bolaamphiphile 4 was fixed to 250 µM.

Therefore, the concentration dependent CD spectra were performed with bolaamphiphile 4 and D/L-tartaric acid. The concentration dependent (0-250 µM) circular dichroism study of 4 only

shows the increment of the peak at 198 nm (Figure S13a). Upon addition of different concentration of D/L-tartaric acid into the solution of 4, the supramolecular chirality changes gradually. The concentration (0-250 µM) of D-tartaric acid and L-tartaric acid were varied in 4. Upon addition of 150 µM of D-tartaric acid, a new positive peak generates nearly about 213 nm (Figure 4). After addition of equimolar amount (250 µM) of D-tartaric acid, complete inversion 14 ACS Paragon Plus Environment

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of the CD spectra is observed with the appearance of two peaks at 194 nm and 213 nm. Therefore, the supramolecular chirality dominates at higher concentration (250 µM) of D-tartaric acid (Figure 4). Bolaamphiphile 4 exhibits a peak at 198 with a negative CD on interaction with L-tartaric acid. The gradual increment of L-tartaric acid (0-250 µM) into the solution of 4 shows

small change in CD spectra even after addition of 250 µM of L-tartaric acid. The CD peaks appeared in the co-assembled system are also different for hydrogels 1 and 2. In case of hydrogel 1, the n-π* transition becomes prominent after co-assembly whereas π-π* transition appears prominent for hydrogel 2. This is attributed to the difference in co-assembly pattern of 4 with D/L-tartaric acid. Therefore it can be concluded that the overall chiral senses of D/L-tartaric acid

are transferred to peptide bolaamphiphile 4 to control the supramolecular chirality when the composite forms hydrogels 1 and 2.50,65 Linear dichroism spectra were recorded to know the detail about induction of supramolecular chirality. In each case, significant change in linear dichroism was observed (Figure S13). Bolaamphiphile 4 shows very less intense negative LD signal.66 After addition of D/L-tartaric acid, supramolecular chirality was induced. The hydrogels 1, 2 exhibit strong LD signals. Hydrogel 1 shows a positive LD peak at 221 nm due to n-π* transition. Hydrogel 2 exhibits prominent negative LD signal at 201 nm due to π-π* transition. The possible reason behind the selective co-assembly of 4 with D/L-tartaric acid is highly dependent on the synergic chirality. The proper complementary functionality is configurationally more fitted to co-assemble 4 with the D/L-tartaric acid whereas other bolaamphiphiles are not suitably fitted with the D/L-tartaric acid. In this case, chiral-chiral selection of L-leucine appended bolaamphiphile 4 synergistically co-assembles with D/L-tartaric acid using hydrogen bonding interactions. D-tartaric acid shows positive CD signals whereas L-tartaric acid exhibits negative CD signals. The dipeptide appended bolaamphiphiles, which contain D-leucine (5-9), do


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not form hydrogel with D/L-tartaric acid. Therefore, 5-9 did not show any change in CD spectra after mixing with D/L-tartaric acid (Figure S14). Conclusion In conclusion, we successfully developed bicomponent hydrogels with controlled right-handed and left-handed twisted fibers. L-leucine based bolaamphiphile 4 co-assembled to form hydrogel. Peptide bolaamphiphiles 5-9 are geometrically not favorable to co-assemble with D-tartaric acid and L-tartaric acids using hydrogen bonding interactions. The overall chiral sense of D/L-tartaric acid was transferred to 4 and nucleation of small chiral unit takes place to control the supramolecular chirality. This nucleation has taken place via the co-assembly between D/Ltartaric acid and 4 through an extensive hydrogen bonding interaction, which was confirmed from FTIR, PXRD and 1H NMR experiments. The small nucleated helical unit again integrated to form narrow helical fibers. It was observed that the position of L/D-leucine is the tuning button to control the overall handedness of aliphatic bolaamphiphiles. The tuning ability will transfer to second amino acids only when the first amino acids act neutrally to the bolaamphiphiles. Lleucine based peptide bolaamphiphile and D/L-tartaric acid synergistically stabilize to form coassembled hydrogels. The nanostructures from sphere to desired left-handed and right-handed nanofibers were successfully tuned in hydrogel state. Supporting Information Synthesis of peptide bolaamphiphiles; corresponding 1H NMR,

13

C NMR and ESI-Mass data; optical

images; SEM and TEM images; rheological data; DLS, FT-IR, PXRD, CD, LD data of different bicomponent systems.

Acknowledgment AKD sincerely acknowledges CSIR, New Delhi, India (project: 01(2936)/18/EMR-II), for financial support. The sophisticated instrumentation centre (SIC), IIT Indore is acknowledged for 16 ACS Paragon Plus Environment

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providing access to the instrumentation. SB is indebted to UGC for his fellowship. SAIF, IIT Bombay is also acknowledged for the assistance of EM facility. Declaration of Interests The authors declare no competing financial interests.


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Table of Content

D-tartaric

L-tartaric

acid

acid

(-) CD

(+) CD

Left

Right

handed

handed


Tuning the Handedness: Role of Chiral Component in Peptide

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